An important and significant goal in healthcare is to discover and make available safer and more effective drugs for the treatment of cancer. Most chemotherapeutic agents act by disrupting DNA metabolism, DNA synthesis, DNA transcription, or microtubule spindle function, or by perturbing chromosomal structural integrity by introducing DNA lesions. These processes affect both normal and tumor cells. The maintenance of DNA integrity is essential to cell viability in normal cells, therefore, anticancer drugs have the lowest therapeutic index of any drug class.
An individual cell creates an exact copy of its chromosomes, and then segregates each copy into two cells by a process called mitosis. The mitotic cycle can be divided into three major events: DNA replication, chromosome segregation, and cell division. Cells have sensing mechanisms to maintain the order of these steps with respect to one another and to ensure that each step is executed with high fidelity. The sensing mechanisms for these processes are referred to as “checkpoints” in L. H. Hartwell et al., Science, Nov. 3, 1989, 246(4930):629-34.
Cell cycle checkpoints have been reported to comprise at least three distinct classes of polypeptides. Each class of polypeptides acts sequentially in response to cell cycle signals or defects in chromosomal mechanisms (Carr, (1996) Science, 271:314-315). One family of proteins detects or senses DNA damage or abnormalities in the cell cycle. These sensors include Ataxia-Telangiectasia Mutated (Atm) and Ataxia-Telangiectasia Rad-related (Atr) (Keegan et al., (1996) Genes Dev., 10:2423-2437). Another class of polypeptides amplify and transmit the signal detected by the detector and is exemplified by Rad53 (Allen et al. (1994) Genes Dev., 8:2416-2488) and Chk1. In addition, cell cycle effectors, such as p53, mediate a cellular response, including, for example, arrest of mitosis and/or meiosis and apoptosis.
DNA damage can be induced by drugs, radiation, or can be spontaneously generated during the course of normal metabolism. DNA damage checkpoints ensure that cells with unrepaired DNA lesions do not progress into the DNA synthesis phase or mitosis until chromosomal lesions have been removed. Cell cycle arrest can enhance the opportunity for DNA repair and increase the fidelity of cell division. DNA damage can be recognized throughout the cell cycle. Checkpoints ensure that the growth of cells is arrested at multiple cell cycle phases. As a result, multiple cell cycle signaling pathways may result during sensitization of cells to DNA damaging agents.
Much of the current understanding of the function of cell cycle checkpoints has been derived from the study of tumor-derived cell lines. In many cases, tumor cells have lost key cell cycle checkpoints (Hartwell et al., Science, Dec. 16, 1994; 266(5192): 1821-8). It has been reported that a key step in the evolution of cells to a neoplastic state is the acquisition of mutations that inactivate cell cycle checkpoint pathways, such as p53. (Weinberg, R. A. (1995) Cell 81:323-330; Levine, A. J. (1997) Cell 88: 3234-331). Loss of these cell cycle checkpoints results in the inappropriate cycling of tumor cells in response to DNA damaging agents. When faced with cellular stresses, such as DNA damage, and cell cycle events with decreased fidelity, tumor cells have difficulty altering the kinetics of cell cycle progression. Therefore, inhibition and disruption of additional DNA damage checkpoint pathways may further sensitize tumor cells to anticancer treatments, such as radiation and chemotherapy.
Noncancerous tissue, which has intact cell cycle checkpoints, typically is insulated from temporary disruption of a single checkpoint pathway. Tumor cells, however, have defects in pathways controlling cell cycle progression such that the perturbation of additional checkpoints, for example, the DNA damage checkpoint, renders them particularly sensitive to DNA damaging agents. For example, tumor cells that contain mutant p53 are defective both in the G1 DNA damage checkpoint and in the ability to maintain the G2 DNA damage checkpoint. (Bunz et al., Science, Nov. 20, 1998; 282(5393): 1497-501; Levine). Checkpoint inhibitors that target initiation of the G2 checkpoint or the S phase checkpoint are expected to further cripple the ability of these tumor cells to repair DNA damage and, therefore, selectively kill them over normal cells. Therefore, checkpoint inhibitors are expected to enhance the therapeutic index, which is a measure of the probability of tumor control relative to the probability of toxicity to normal tissue, of both radiation and systemic chemotherapy.
The ability of checkpoint inhibitors to enhance the therapeutic index may be dependent upon tumor type. Tumors with cell cycle defects complementary to the DNA damage checkpoint pathways may be most sensitive to inhibitor drug treatment. In contrast, DNA-PK inhibitors, another distinct class of potential therapeutic agents, are expected to sensitize tumors independently of cell type. A systematic approach of applying checkpoint inhibitors and DNA-PK inhibitors also may be effective in the treatment of metastatic diseases that radiation therapy cannot target.
The checkpoint proteins Atm and Atr are hypothesized to initiate a signal transduction pathway leading to cell cycle arrest in the presence of DNA damage or any block to DNA replication. Atm has been shown to play a role in a DNA damage checkpoint in response to ionizing radiation (IR). Patients lacking functional Atm develop the disease Ataxia-Telangiectasia (A-T). Symptoms of A-T include extreme sensitivity to ionizing radiation (IR), cerebellar degeneration, oculotaneous telangiectasias, gonadal deficiencies, immunodeficiencies and increased risk of cancer (Shiloh, Eur. J. Hum. Genet 1995; 3(2):116-38). Fibroblasts derived from these patients are thought to have defects in G1, S, and G2 checkpoints and are defective in their response to IR (Kastan et al., Cell, Nov. 13, 1992; 71(4): 587-97; Scott et al., Int. J. Radiat. Biol., December, 1994; 66(6 Suppl): S157-63; and Beamish et al., J. Biol. Chem. Aug. 26, 1993; 271(34):20486-93). Therefore, Atm may sense double-strand DNA damage caused by IR and radiomimetic drugs, and signal the cell cycle to arrest, such that damage can be repaired.
Atr is a checkpoint protein stimulated by agents that cause double strand DNA breaks, single strand DNA breaks, and agents that block DNA radiation. Overexpression of Atr in muscle cells on 2.0 iso-chromosome 3q results in a block to differentiation, abnormal centrosome numbers, chromosome instability, and abolishes the G1 arrest in response to IR (Smith et al., Nat. Genet., May 1998; 19(1): 39-46). Overexpression of a kinase inactive, dominant negative mutant of Atr sensitizes cells to IR, ultraviolet light (UV), MMS, and cisplatin (cliby et al., EMBO J. Jan. 2, 1998, 17(1):159-69 and Wright et al., Proc. Nat'l Acad. Sci. U.S.A., Jun. 23, 1998; 95(13):7445-50). Cells containing overexpressed, mutant strain Atr also fail to arrest in G2 in response to IR. In addition, Atr is associated with chromosomes in meiotic cells where DNA breaks and abnormal DNA structures persist as a result of the process of meiotic recombination (Keegan et al., Genes Dev. Oct. 1, 1996; 10(19): 433-37). Atr, like Atm, also senses DNA damage and agents that block DNA replication, as well as initiates a cell cycle arrest at G2 and S for DNA repair.
Chk1 is hypothesized to lie downstream from protein kinases Atm and/or Atr in the DNA damage checkpoint signal transduction pathway. (See, Sanchez et al., Science, 1997; 277:1497-1501; U.S. Pat. No. 6,218,109.) In mammalian cells, Chk1 is phosphorylated in response to agents that cause DNA damage including IR, UV, and hydroxyurea (Sanchez et al., 1997; Lui et al., Genes Dev. 2000; 14:1448-1459). The phosphorylation and activation of Chk1 in mammalian cells is dependent on Atm (Chen et al., 1999) and Atr (Lui et al., 2000). In the yeast S. pombe, Chk1 also appears to be involved in the response to IR and blocks to replication (Boddy et al., 1998; Walworth et al., 1993). Furthermore, Chk1 has been shown to phosphorylate both weel (O'Connell et al., EMBO J. 1997; 16:545-554) and Pds1 (Sanchez et al., Science 1999; 286:1166-1171) gene products known to be important in cell cycle control. These studies demonstrate that mammalian Chk1 plays a role in both the Atm-dependent DNA damage checkpoint leading to arrest at S phase. However, a role for Chk1 in the S phase replication checkpoint in mammalian cells has yet to be elucidated. Interestingly, Chk1 knockout mice are embryonically lethal, thereby suggesting a role for Chk1 in a developing organism in addition to its role in DNA damage checkpoints.
Chk1 may invoke a G2 arrest by phosphorylating and inactivating Cdc25C, the dual specificity phosphatase that normally dephosphorylates cyclin B/cdc2 as cells progress into mitosis (Fernery et al., Science, Sep. 5, 1997; 277(5331): 1495-7; Sanchez et al.; Matsuoka et al.; and Blasina et al., Curr. Biol., Jan. 14, 1999; 9(1):1-10). This mechanism of regulation of Cdc2 activity stimulates cell cycle arrest to prevent cells from entering mitosis in the presence of DNA damage or unreplicated DNA.